Hearing aid fitting is a process of adjusting the overall gain, the frequency response, and dynamic processing parameters of an electronic hearing instrument to best match the requirements of an individual user. The fitting process is generally carried out by a hearing professional, such as an audiologist, an ear, nose, and throat doctor, or a hearing aid dispenser. Hearing aid fitting is usually based on a number of diagnostic tests which are performed as part of, or prior to the fitting session. These diagnostic tests may include a threshold audiogram, and tests to establish the most comfortable (MCL) and uncomfortable (UCL) listening levels in different frequency bands. These tests are usually administered using standard audiometers which present pure test tones and bands of noise at different frequencies and different amplitudes. These sounds are presented to the test subject through headphones or in free space from loudspeakers. The test subject responds to the presentation sounds by indicating whether the sound is barely audible, as in the case of threshold tests, or with a judgment about the loudness of a sound, as in the case of the MCL and UCL tests.
The result of these diagnostic tests is often a prescription for a hearing aid having an insertion gain (IG) which specifies the desired frequency dependent gain that a hearing aid delivers to provide maximum satisfaction for the hearing aid user.
Some hearing aids provide dynamic range compression in which the gain applied in a given frequency band can be a function of the amount of power in that frequency band. This may be viewed as different insertion gains for different input power levels. Compressing hearing aids have a number of time constants which determine how quickly the insertion gain changes as a function of changes of input power level. A prescription for a compressing hearing aid may include multiple frequency dependent insertion gains or a formula for modifying a single frequency dependent insertion gain based on input power--compression ratios are a way to express this--and associated compression time constants.
A number of formulae have been devised which receive as input the result of a set of diagnostic tests and produce as an output a hearing aid prescription. An example of this is the Australian National Acoustics Laboratory (NAL) formula for noncompressing aids. Systems for fitting compressing aids from loudness judgment test data are described in Fred Waldhauer et al., "Full Dynamic Range Multiband Compression In A Hearing Aid", The Hearing Journal, September 1988, at 1-4 and U.S. Pat. No. 4,718,499.
Given a hearing aid prescription, an important goal of the fitting session is to adjust the hearing aid to achieve the prescription. A limitation of performing this adjustment is that the frequency response and gain of a hearing aid can only really be determined when it is plugged into the ear. This is because the ear canal, eardrum, the degree to which the hearing aid seals the ear canal, and variations from one hearing aid device to another, all affect the frequency response of the aid. To overcome this limitation the hearing aid fitter often uses a probe microphone which is a microphone in the form of a very fine flexible tube which can be inserted into the ear canal with the tip of the tube placed near the eardrum while the hearing aid is in place. The probe microphone then measures the energy present at the eardrum. Another microphone is generally placed just outside the ear to determine the energy arriving at the input of the hearing aid. With these two measurements, the overall gain and frequency response characteristics of the hearing aid can be determined.
The probe microphone measurement approach to hearing aid fitting is susceptible to various causes of measurement errors. These include pinching of the probe microphone tube, variability in placement of the tip of the tube in relation to the eardrum, and plugging of the tube with earwax, dirt or debris. These problems make probe tube measurements difficult and time consuming.
Even if the hearing aid prescription has been successfully implemented, there is still no guarantee that the hearing aid has been adjusted for maximum satisfaction. This is because the formulae which have been used to determine the hearing aid prescription cannot account for the myriad subjective factors which govern hearing aid acceptance. As a result, after implementation of a hearing aid prescription, the fitting session may continue with the hearing aid fitter applying a number of artful manual readjustments of the hearing aid response. To aid in this process, the hearing aid fitter often presents a selection of real world sounds which the test subject listens to through loudspeakers in an attempt to simulate various listening environments. The hearing aid fitter then interrogates the subject about the quality of the sounds and uses the responses as a guide to further readjustment.
A problem relating to this readjustment process is that presentation of sounds through loudspeakers must be done in a controlled and repeatable way so that, for example, sounds which are supposed to be perceived as being at conversational level are indeed presented at this level. This means that the placement of the loudspeakers, the amplification system, and the distance and orientation of the subject in relation to the loudspeakers all must be properly controlled.
It is desired to have a hearing aid that alleviates the problems associated with traditional fitting using probe microphones and external loudspeakers.